Analyzer employing magneto-optic rotation

ABSTRACT

A magneto-optic rotation analyzer method and apparatus is disclosed. In the analyzer, a light beam of linearly polarized light, preferably in the ultra-violet region is shown through a sample of a material to be analyzed. A magnetic field is applied to the sample parallel to the light beam to obtain magneto-optic rotation of the polarization of the light by constituents of the sample. The emerging light beam is analyzed as to its polarization to separate light which is rotated from light which has not been rotated. One of the separated light beams is detected by a photodetector. The magneto-optic rotation effect is modulated at a certain modulation frequency and the output signal is synchronously detected against the modulation frequency or a harmonic thereof for improved signal-to-noise ratio. The synchronously detected output is measured to obtain a measurement of the sample under analysis.

[ June 12, 11973 ANALYZER EMPLOYING Primary Examiner-John K. CorbinMAGNETO-OPTIC ROTATION Attorney-Roland I. Griffin [75] Inventors: RobertL. Chaney, Cupertino;

Michael A. Kelly, Mountain View, [57] ABSTRACT b h of C lif Amagneto-optic rotation analyzer method and apparatus is disclosed. Inthe analyzer, a light beam of linearly [73] Asslgnee: Hewlett'lfackardCompany Palo polarized light, preferably in the ultra-violet region isAlto Calif shown through a sample of a material to be analyzed. 22Filed; July 14, 1971 A magnetic field is applied to the sample parallelto the light beam to obtain magneto-optic rotation of the po- [21] Appl'162502 larization of the light by constituents of the sample. Theemerging light beam is analyzed as to its polarization to [52] [1.8. CI356/117, 250/225, 350/149 Separate light which is rotated from lightwhich has not [51] Int. Cl. G0ln 21/40 been rotated- One of theseparated light beams is [58] Field of Search 356/114, 117; tected y aphotodetector- The magneto-optic rotation 250/225 effect is modulated ata certain modulation frequency and the output signal is synchronouslydetected against 5 R f r n Cit the modulation frequency or a harmonicthereof for UNITED STATES PATENTS improved signal-to-noise ratio. Thesynchronously detected output is measured to obtain a measurement of3,442,592 5/1969 Gros ean.. 356/ll4 X 3,450,477 6/1969 Meltzer 356/114the sample under analyss' 9 Claims, 7 Drawing Figures SOLENOID x- LINEARx-Lmr/m LAMP POLARIZEAR V POLARIZER l8 PHOTOMULTIPLIER 2 6 E CH 7 19 4f1 CELL g 7 e B 8 l z l? y- 21 FILTER 1 9 I ll I w; wi SYNCHRONOUSoscumon DOUBLE DETECTOR READOUT PAIEHIEI] .III I 2 I975 SHEET 1 0F 3SOLENOID X-LINEAR X-LINEAR LAMP POLARIZEAR POLARIZER 8 PHOTOHULTIPLIER 261. H 17 19 FI W 21 FILTER 1 9 I & I

II w] SYNCHRONOUS OSCILLATOR DOUBLER DETECTOR READOUT 32 i9ure I T 3LAMP PROFILE I 12 I fW/INP PROFILE LESS COMPONENTS ROTATED BY: No.S02.CH20+ N02 I N0 2 CH2) MAGNETIC ROTATION SPECTRA INVENTORS ROBERT LvCHANEY figure 3 MICHAEL A KELLY BY WNW ATTORNE PAINTED- 3.738.755

SHKEIZUFS Q PULSE AMPLIFIER 9 as j i w 4 DECADE up-oowu TRANSFER &BUFFER OSCILLATOR DOUBLER 2w 7 COUNTER STORAGE I 2 L26 L 3? V r 39 0mmDISPLAY Tigure 4 A? 2 4 Z 7 5 4e 45 E TUNABLE R.F.

OSCILLATOR -26 F 29 r r 51 32 1 L SENSITIVE OSCILLATOR DOUBLER DETECTORi9ure 5 INVENTORS ROBERT L CHANEY MICHAEL A, KELLY ATTORNEY PAIENIEU JW1 2 SWEET 3 BF 3 4 n L\-J'146 45 48 YR X k SENSFTTVE n TUNABLE R'E L49DETECTOR OSCILLATOR I W) 1 26 29 32 l (b) w 3; L, OSCILLATOR M DOUBLERWm T i9ure 6 MAGNETIC FIELD POLARlZATION TIME Uure 7 INVENTORS ROBERT L.CHANEY MICHAEL A. KELLY ATTORNEY I ANALYZER EMPLOYING MAGNETO-OPTICROTATION BACKGROUND OF THE INVENTION I-Ieretofore, magneto-opticrotation monochromator or light filter experiments have been disclosedwherein the magnetic field within a known sample has been modulated. Thepolarization analyzed output beam from the sample was fed through anextremely narrow optical band resonance cell filter and thence to aphotodetector. Such a magneto-optic monochromator is disclosed in anarticle titled Modulation and Filtration of Resonance Radiation With TheUse Of The Faraday Effect appearing in Optics and Spectroscopy, Volume19, No. 3, pages 254-255, of September 1965. While this monochromator ofthe prior art is useful for examining qualitative line splittings bymagneto-optic rotation near the edges of a relatively narrow intenseknown spectral line, it is not used as an analyzer for determiningquantitative information concerning the amount of trace quantities ofunknown molecular or atomic constituents of fluid samples, such as gasor liquid samples, particularly where the signal-to-noise ratio isorders of magnitude below the signal-to-noise ratio in theaforementioned prior art experiment.

In another prior art monochromator experiment the magneto-optic activitynear the band edges of known strong absorption lines of a known gas hasbeen qualitatively and quantitatively investigated. The band edge linesplittings due to Zeeman splitting and transitions between rotationaland vibrational states of the molecule or atom have been studied andrelated to the molecular or atomic structure. In such an experimentalapparatus it was speculated that more easily interpretable results couldbe obtained by modulating the applied longitudinal magnetic field andemploying phasesensitive detection. See an article titled MagneticOptical Activity, appearing in the Annual Review of Physical Chemistry,Vol. 17, (1966), pp. 399-432 at p. 427. However, this work does notcontemplate a liquid or gas analysis system for obtaining quantitativeinformation about the amount of trace quantities of a gas in a sampleunder analysis.

SUMMARY OF THE INVENTION The principalobject of the present invention isthe provision of an improved analyzer employing magnetooptic rotation.

One feature of the present invention is the provision,

I in a magneto-optic rotation analyzer of modulating the nal, whereby ameasurement of the synchronously detected output yields a measure of thequantity of a magneto-optically active material within the sample.

Another feature of the present invention is the same as the precedingfeature wherein the magneto-optic rotation effect caused by the sampleis modulated by modulating the intensity of the magnetic field appliedto the sample.

Another feature of the present invention is the same as the firstfeature wherein the magneto-optic rotation effect is modulated bymodulating the frequency of the probing light beam.

Another feature of the present invention is the same as any one or moreof the preceding features including the provision of an optical filterdisposed between the source of the probing light and the opticaldetector for limiting the passband of the optical light passing to theoptical detector to a band encompassing substantially only themagneto-optic rotation spectrum of a constituent of the sample to bedetected.

Another feature of the present invention is the same as the precedingfeature wherein the optical passband of the probing light passed by thefilter to the optical detector is changed from one band to another fordetecting different constitutents within the sample.

Another feature of the present invention is the provision, in amagneto-optic rotation molecular analyzer, of directing a probing lightbeam through the sample under analysis, such light beam having abandwidth of optical frequencies in excess of the bandwidth of themagneto-optic rotation spectrum of a constituent of the sample to bedetected, scanning the frequency of the light passed to thephotodetector to repetitively scan through the magneto-optic rotationspectrum of the constituent of the sample to produce modulation of thelight detected by the optical detector at a frequency related to thescanning frequency, and detecting the output electrical signal of theoptical detector against a reference frequency related to the scanfrequency to separate the magneto-optic rotation signal from thecomposite detected electrical signal.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the composite electrical signal isdetected against the reference frequency by counting the signal upduring one portion of the reference cycle and counting the signal downduring another portion of the reference cycle.

Other features and advantages of the present invention will becomeapparent upon perusal of the following specification taken in connectionwith the accompanying drawings wherein:

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic line diagram, partlyin block diagram form depicting a magneto-optic rotation analyzerincorporating features of the present invention.

FIG. 2 is a plot of detected optical intensity I versus opticalwavelength for three quantities, namely, lamp profile, lamp profile lessabsorption by certain gases, and for the magnetic rotation spectrum ofthe various constituents of the gas sample.

FIG. 3 is a plot of the waveforms versus time for the applied magneticfield, the magneto-optic polarization rotation, and the output of thephotomultiplier detector for the analyzer of FIG. 1.

FIG. 4 is a schematic block diagram for a magnetooptic rotation analyzersimilar to that of FIG. 1 and employing alternative features of thepresent invention.

FIG. 5 is a schematic block diagram for a magnetooptic rotation analyzeremploying alternative features of the present invention.

FIG. 6 is a schematic line diagram, partly in block diagram form,depicting a magneto-optic rotation analyzer incorporating alternativefeatures of the present invention.

FIG. 7 is a plot of waveforms for magnetic field and polarizationrotation as a function of time and depicting an alternative method ofmodulation employed in the analyzers of FIGS. 1, 4 and 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown the magnetooptic rotation molecular analyzer 1, incorporatingfeatures of the present invention. The analyzer includes a light sourceor lamp 2, such as a deuterium lamp producing ultra-violet radiationhaving a spectral profile of intensity versus wavelength as shown bycurve 3 of FIG. 2. The light from the lamp 2 is directed through alinear polarizer 4, such as a calcite or quartz Gian-Taylor prism, intoa sample cell 5, such as a stainless steel bobbin, of 0.5 inch insidediameter and 4.0 inches long, having quartz windows 6 and 7 closing offopposite ends of the cell. Fluid to be analyzed, such as a gas orliquid, is fed through the cell via input line 8 and output line 9. Amagnetic solenoid 11 is wound on the bobbin for producing a relativelystrong axial magnetic field I! having a strong component directedparallel to the path of the light beam passing through the sample cell5.

The action of the magnetic field H on the molecules within the samplefluid being analyzed is to rotate the polarization of the light suchthat a small fraction of the light is rotated 90 relative tothe linearpolarization of the light beam incident on the sample. The light whichis rotated by the sample constituents is found to have occurred atdifferent wavelengths across the spectral profile of the lamp. Forexample, in FIG. 2, curve 12 depicts the lamp profile less thosecomponents of the light beam which are rotated by 90 due to passingthrough the sample constituents consisting of NO, S CH O and N0 Thespectral profiles for the various components of light which have beenrotated by 90 are indicated by curves 13, 14, 15, and 16 of FIG. 2.

The light beam containing the rotated components is passed through asecond linear polarizer 17 with the direction of polarization oriented90 relative to the orientation of the input linear polarizer 4. Thesecond linearpolarizer 17, which is essentially identical to the firstlinear polarizer 4, serves to separate the light components 20 whichhave been rotated by 90 relative to those components 21 which have notbeen rotated. Thus, the output of linear polarizer 17 consists of afirst beam 20 which is passed through a filter 18 to a photomultiplier19. Beam 20 consists of the rotated components similar to thoseindicated by curves 13-16 of FIG. 2. On the other hand, the secondlinear polarizer 17 directs the remaining lamp profile light asindicated by curve 12 of FIG. 2, into the second beam 21. Either one ofthe output beams 20 or 21 may be directed to the photomultiplier 19 byshifting by 90 the orientation of the output linear polarizer 17. Eachlight beam 20 and 21 contains essentially the same information, however,beam 20 has improved signal-to-noise ratio.

The filter 18 between the analyzer 17 and the photomultiplier 19 ispreferably selected to have an optical passband covering essentiallyonly the magnetorotation spectrum of the sample constituent to bedetected. For example, if it were desired to detect NO constituents inair, filter 18 would have an optical passband approximately 30 angstromswide centered at 2,260 angstroms. If it were desired to detect S0 filter18 would have a passband of 200 angstroms centered at 3,200 angstroms.If it were desired to detect N0 filter 18 would have a passband ofapproximately 1,000 angstroms centered at 5,000 angstroms.

The magnetic field H is preferably an alternating magnetic field at asuitable audio frequency, such as 270 Hertz, as indicated by waveform 25of FIG. 3. The alternating magnetic field H has a peak-to-peak amplitudeof 2 kilogauss alternating between plus and minus 1,000 kilogauss and asinusoidal waveform of frequency m supplied by a power oscillator 26which feeds energizing current to solenoid 11. On each half cycle of theapplied magnetic field H, the magnetooptic rotation of the polarizationof the light passing through the cell, as operated on by the sampleconstituents, has a peak amplitude as shown by curve 27 of FIG. 3.Although the peak rotation of the polarization is shown as plus andminus 10 a certain fraction of the light that is shifted in polarizationwill be shifted by and the waveform for the 90 shift will be similar tothat of waveform 27.

The output linear polarizer 17 converts waveform 27 into waveform 28with one peak for each half cycle of the applied alternating magneticfield. As a result, each of the magneto-optic rotation spectrums foreach of the sample components is pulsed on for each half cycle of theapplied alternating magnetic field. The spectrum which is passed throughthe filter 18 and which is pulsating at twice the frequency of theapplied magnetic field 20), produces a time-varying signal of frequency20), in the photomultiplier 19 having a waveform as shown by curve 28 ofFIG. 3. Thus, it is seen that the composite output electrical signalfrom photomultiplier 19 includes a component 200 at twice the mdoulationfrequency w, and this component corresponds to the signal derived fromthe selected magneto-optic rotation spectrum within the sample underanalysis.

The magneto-optic rotation signal component is separated from thecomposite signal 28 by synchronous detection correlation against areference signal of the same frequency 20), as the signal componentwithin the output of the photomultiplier 19. In the example given, theoutput of the w, oscillator is sampled and fed to a doubler 29 whereinit is doubled to provide a 2m, reference fed to one input of asynchronous detector 31 for synchronous detection of the output signal28 to separate the desired magneto-optic rotation spectrum signal fromthe composite signal. The output of the synchronous detector is fed to asuitable readout 32, such as an integrator and meter, indicated at 32 toobtain a measure of the quantity of the selected sample constituent,such as NO, 80,, CH O, or N0 Synchronous detector 31 may take severalforms, such as a phase sensitive detector, a synchronous switch or thelike.

Referring now to FIG. 4, there is shown an alternative molecularanalyzer 35 substantially the same as that previously described withregard to FIG. 1 with the exception that a pulse amplifier 36 and an upand down counter 37 have been substituted for the synchronous detector31. Pulses corresponding to the arrival of individual photons are thusdetected so the analyzer of FIG. 4 is specially useful for detectingextremely low concentrations of constituents of the sample underanalysis. For example, the system of FIG. 4 is useful I over the rangeof concentration from 10 to 10" of the constituent in the sample. In therelatively low concentration ranges E a relatively large background ofnoise pulses are produced in the output of the photomultiplier ll9 andthe signal which it is desired to detect can be much smaller than thenoise. For example, approximately lO spurious pulses per second areproduced in the output of the photomultiplier 19, whereas the signal mayproduce only approximately I00 pulses per second. Thus, it is seen thatthe signal is about two orders of magnitude lower in intensity than thenoise out of which it must be detected.

The output of the photomultiplier 19 comprises a series of pulses withpulse widths of approximately 40 nanoseconds each and with a repetitionrate varying from the low end of the range to the high end of the rangefrom 1O to 100 megahertz. The pulses from the output of thephotomultiplier 19 are fed to a pulse amplifier 36 wherein they areamplified. The pulse amplifier 36 preferably has a relatively wide bandfor amplifying the relatively short pulses, and, in addition, has a logtype gain characteristic such that low intensity pulses are amplifiedconsiderably more than high intensity pulses. In this manner, the outputpulses are relatively constant amplitude for counting in the up-downfour decade counter 37.

At the low end of the detection range of the analyzer 35, the output ofthe pulse amplifier 36 consists of a background of approximately 10pulses per second. Superimposed upon this 10 pulses per second is a timevariation of 100 pulses per second produced by the sample. The timevarying increase in the pulse rate occurs twice per cycle of thealternating magnetic field of frequency m Thus, the output of thedoubler 29, which consists of the 2m, reference, is fed to one input ofthe up-down counter 37 such that the counter counts in one direction,such as up, when the signal is present and counts down the remaininghalf cycle of the reference when the signal is absent. In this manner,the noise is automatically substracted from the composite signal toseparate the desired magneto-optic rotation from the noise. As the countcontinues, the signal is automatically integrated. After a predeterminedduration of the count, the output of the counter 37 is fed to a transferand buffer storage 38 and displayed on a digital display 39 to yield ameasure of the concentration of the selected constituent in the sample.

Upon receipt of the integrated count from the counter, the transfer andbuffer storage 38 feeds a signal back to the up-down counter 37 to starta new count cycle. The duration of the count is determined by a timer inthe transfer buffer storage 38 and the count time is set by the operatorand will depend upon the sig nal level. The count time is selected togenerally count a certain predetermined number of pulses at apredetermined expected pulse rate depending upon the expectedconcentration of the sample constituent and may vary from 0.01 second tominutes.

Referring now to FIG. 5, there is shown a magnetooptic rotationmolecular analyzer 43 incorporating alternative features of the presentinvention. Analyzer 43 is similar to that described with regard to FIG.1 with the exception that the optical filter 18 is replaced by an RFtunable acousto-optic filter 44 disclosed in an article titledAcousto-Optical Tunable Filter appearing in the Journal of the OpticalSociety of America, Vol. 59, No. 6 of June, 1969, pages 744-747, and inan article titled Electronically Tunable Acousto-Optic Filter appearingin the Applied Physics Letters, Vol. 15, No. 10 of Nov. 15, 1969, pages325 and 326.

The acousto-optic filter 44 is electronically tunable over a relativelywide band and has an instantaneous passband which can be designed tohave a passband width from a fraction of an angstrom to a few hundredangstroms. The acousto-optic filter 44 includes a optically birefringentcrystal 45, as of lithium niobate. An acoustic transducer 46 is coupledto the crystal 45 near the upbeam end thereof for producing an acousticwave in the cyrstal which is reflected from the input face 47 along theaxis of the crystal 45 collinear with the light beam passed therethroughfor cumulative collinear diffraction for diffracting light within thepassband of the filter 44 from a first polarization, as produced bypolarizer 4, into an output beam of orthogonal polarization. An outputpolarization analyzer 49, such as a Glan-Taylor prism, analyzes theoutput beam of the filter to separate the crossed polarized light fromthe light of the input polarization. The crossed polarized light, withinthe passband of the filter, is fed into the sample cell 5. Lightcomponents, within the light beam fed into the sample cell 5, which arewithin the spectral bandwidths of the sample constituents aremagneto-optically rotated and the output polarization analyzer 17separates those light components which have been rotated from thosecomponents hwich have not been rotated and feeds the rotated componentsto the photo-multiplier detector 19.

In one mode of operation, a tunable RF oscillator 49, which generatesthe electrical excitation for the transducer 46, is tuned to a frequencysuch that the passband of the optical filter 44 is centered on themagnetooptic rotation spectrum for the sample constituent to beobserved. The passband frequencies of the optical filter 44 are thenrepetitively scanned back and forth across the spectrum of the samplecomponent to be detected at the scan frequency al as determined by theoutput of the w, oscillator 26 fed to the tuning control input of theoscillator 49. This produces a 200 signal component in the compositeelectrical signal in the output of the photomultiplier 19 which is fedto one input of a phase sensitive detector 51 for detection against a20), reference signal derived from the output of the doubler 29 toseparate the magneto-optic rotation signal from the noise. The outputsignal is a DC output in the output of the phase sensitive detector 51.The DC signal output is fed to the meter 32 and read out for giving ameasure of the sample constituent to be observed within the sampel underanalysis.

Referring now to FIG. 6, there is shown another magneto-optic rotationanalyzer 53 incorporating features of the present invention. Theanalyzer 53 is similar to analyzer 43 of FIG. 5 with the exception thatthe acousto-optic tunable filter 44 is positioned between the outputpolarization analyzer 17 and the photomultiplier detector 19. The outputanalyzer 17 may have its polarization set the same as the inputpolarizer 4 for detecting the lamp profile signal less the rotatedcomponents, as indicated by waveform (a), or it may be cross polarizedto the input polarizer 4 for detecting the magnetooptic rotationspectrum, as indicated by waveform (b). The passband of theacousto-optic filter 44 is then set to the center of the magneto-opticrotation spectrum of the sample constituent to be analyzed and thepassband of the acousto-optic filter 44 is repetitively scanned, at afrequency oi over the spectrum of the sample constituent to be detectedto produce an output signal at twice the modulation frequency 2m, in thecomposite electrical signal output of the photomultiplier 19, as shownby waveform (c) or d). The composite signal is then phase sensitivedetected against a reference signal 2w, derived from the output of thedoubler 29 to produce the output signal which is integrated and measuredby meter 32 to give a measure of the quantity of the constituent of thesample under analysis.

In the various spectrometer embodiments of FIGS. 1, 4, 5, and 6 eitherthe magneto-optic rotation produced by the sample component to bedetected is modulated or the magneto-optic rotation spectrum of thecomponent to be detected is repetitively scanned in such a manner as toproduce two output pulses for each scan cycle. Therefore, theaforedescribed synchronous detection schemes employed a doubler 29 fordoubling the modulation frequency to derive the reference signalemployed in the synchronous phase sensitive detector.

In an alternative embodiment, either the magnetooptic rotation effect orthe magneto-optic rotation spectrum is scanned in such a manner as toproduce only one magneto-optic rotation output pulse per cycle of themodulation, therefore the requirement for the doubler can be eliminatedin the various synchronous detection schemes disclosed in thespectre-meters of FIGS. 1, 4, 5, and 6. More specfically, in theembodiments of FIGS. 1 and 4, the magnetic field may be swept by asaw-tooth waveform such that only one shift in the polarization isobtained per cycle of the modulation, as indicated by waveforms 56 and57 of FIG. 7. Likewise, in the molecular analyzers 43 and 53 of FIGS.and 6, the passband of the acousto-optic filter 44 can be set to oneside of the magneto-optic rotation spectrum to be observed. Then eithera sinusoidal or saw-tooth modulation is applied to the scan to produce amagneto-optic rotation signal having a strong component at themodulation frequency w, derived from the w, oscillator 26 or the to,scan generator. In addition, the up-down counter type of synchronousdetection of FIG. 4 may be employed in the spectrometers of FIGS. 5 and6.

Although, as thus far described, the magneto-optic analyzers of thepresent invention have been described for analyzing for moleuclarconstituents, the method and apparatus of the present invention is alsoused to advantage for detecting atomic constituents such as Hg, Cd, Na,K, etc.

Since may changes could be made in the above construction and manyapparently widely different embodiments of the invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

We claim:

1. In a method for analyzing a sample by detection of magneto-opticrotation the steps of, disposing a sample of matter to be anaylzed toreceive a probing light beam having light of a first polarization,applying a magnetic field to the sample with a substantial component ofthe applied magnetic field being directed along the direction of thepath of propagation of the probing light beam within the sample toproduce magneto-optic rotation of the polarization of the light by thesample from the first polarization to a second polarization, saidmagnetic field comprising an alternating magnetic field, whereby saidalternating magnetic field produces modulation of the magneto-opticrotation at a frequency which is a multiple of the frequency of theapplied alternating field, and including the step of, generating areference signal at a frequency which is a multiple of the frequency ofthe applied alternating magnetic field, analyzing the polarization ofthe light emerging from the sample, as affected by sample, to separatethe emerging probing light of the first polarization, detecting theseparated light of one of said polarizations to obtain a compositeelectrical output signal having a time varying magneto-optic electricalsignal component of a frequency which is a multiple of the frequency ofthe applied alternating field and of an amplitude which is responsive tothe quantity of material within the sample which is magneto-opticallyactive within a band of optical frequencies of the probing lightincident on the detector, corelating the composite electrical signalagainst said reference signal to obtain an output proportional to thetime varying magneto-optic r0- tation electrical signal component andseparated from the remainder of the composite electrical singal, andmeasuring the amplitude of the separated output signal to obtain ameasure of the quantity of material within the sample which ismagneto-optically active within the band of optical frequencies of lightdetected by the detector.

2. The method of claim 1 wherein the probing light incident on thesample has a first optical bandwidth broader than the magneto-opticrotation band of a component to be detected in the sample, and includingthe step of, filtering the probing light emerging from the sample at apoint between the sample and the optical detector for passing to theoptical detector only light within an optical passband less than thebandpass of the incident probing light and encompassing substantiallyonly the magneto-optic rotation spectrum of the sample constituent to bedetected.

3. The method of claim 2 including the step of, changing theopticalpassband frequencies of the light passed by the filter to the opticaldetector for detecting magneto-optic rotation of different components ofthe sample.

4. In an apparatus for analyzing a sample by detection of magneto-opticrotation, means for probing a sample of matter to be analyzed with alight beam having light of a first polarization, means for applying amagnetic field to the sample with a substantial component of appliedmagnetic field being directed along the direction of the path ofpropagation of the probing light beam within the sample to producemagneto-optic rotation of the polarization of the light by the samplefrom the direction of the first polarization to the direction of asecond polarization, said magnetic field comprising an alternatingmagnetic field, whereby the alternating magnetic field produces amodulation of the magneto-optic rotation at a frequency which is amultiple of the frequency of the applied alternating magnetic field,means for generating a reference signal at a frequency which is amultiple of the frequency of the applied alternating magnetic field,means for analyzing the polarization of the light emerging from thesample as affected by the sample to separate the emerging light of tjefirst polarization from the emerging light of the second polarizaiton,means for detecting the separated light of one of said polarizations toobtain a composite electrical signal having a time varying magneto-opticrotation electrical component of a frequency which is a function of thefrequency of the modulation of the magneto-optic rotation and of anamplitude which is responsive to the quantity of material within thesample which is magneto-optically active within the band of opticalfrequencies of light incident of said detector, means for correlatingthe composite electrical signal against said reference signal to obtainan output proportional to the time varying magneto-optic rotationelectrical component separated from the remainder of the compositeelectrical signal, and means for measuring the amplitude of the outputto obtain a measure of the quantity of material within the sample whichis magneto-optically active within the band of optical frequencies ofthe light detected by the said detector.

5. The apparatus of claim 4 wherein the probing light beam has a firstoptical bandwidth broader than the magneto-optic rotation band of acomponent to be detected in the sample, and including means forfiltering the light emerging from the sample at a point between thesample and said light detector means for passing to said light detectormeans, only light within an optical passband narrower than the bandwidthof the incident probing light and encompassing substantially only themagneto-optic rotation spectrum of the sample constitutent to bedetected.

6. The apparatus of claim 5 including means for changing the opticalpassband of frequencies of the light passed by said filter to said lightdetector for detecting magneto-optic rotation of different componentswithin the sample.

7. The apparatus of claim 4 wherein said means for detecting thecomposite electrical signal against said reference signal includessynchronous detector means for synchronously detecting the compositesignal against the reference.

8. The apparatus of claim 4 wherein said means for detecting thecomposite electrical signal against said reference signal comprises aphase sensitive detector means.

9. The apparatus of claim 4 wherein said means for detecting thecomposite electrical signal against said reference signal comprises,counter means for counting in one direction during certain time periodsof the reference quantity and for counting in the opposite directionduring remaining periods of the reference quantity and means forsubtracting counts in one direction from the counts in the otherdirection to derive a difference count, said difference countcorresponding to an integration of the time varying electricalmagneto-optic rotation signal component which is separated from theremainder of the composite electrical signal.

1. In a method for analyzing a sample by detection of magnetoopticrotation the steps of, disposing a sample of matter to be anaylzed toreceive a probing light beam having light of a first polarization,applying a magnetic field to the sample with a substantial component ofthe applied magnetic field being directed along the direction of thepath of propagation of the probing light beam within the sample toproduce magneto-optic rotation of the polarization of the light by thesample from the first polarization to a second polarization, saidmagnetic field comprising an alternating magnetic field, whereby saidalternating magnetic field produces modulation of the magnetoopticrotation at a frequency which is a multiple of the frequency of theapplied alternating field, and including the step of, generating areference signal at a frequency which is a multiple of the frequency ofthe applied alternating magnetic field, analyzing the polarization ofthe light emerging from the sample, as affected by sample, to separatethe emerging probing light of the first polarization, detecting theseparated light of one of said polarizations to obtain a compositeelectrical output signal having a time varying magneto-optic electricalsignal component of a frequency which is a multiple of the frequency ofthe applied alternating field and of an amplitude which is responsive tothe quantity of material within the sample which is magneto-opticallyactive within a band of optical frequencies of the probing lightincident on the detector, corelating the composite electrical signalagainst said reference signal to obtain an output proportional to thetime varying magneto-optic rotation electrical signal component andseparated from the remainder of the composite electrical singal, andmeasuring the amplitude of the separated output signal to obtain ameasure of the quantity of material within the sample which ismagnetooptically active within the band of optical frequencies of lightdetected by the detector.
 2. The method of claim 1 wherein the probinglight incident on the sample has a first optical bandwidth broader thanthe magneto-optic rotation band of a component to be detEcted in thesample, and including the step of, filtering the probing light emergingfrom the sample at a point between the sample and the optical detectorfor passing to the optical detector only light within an opticalpassband less than the bandpass of the incident probing light andencompassing substantially only the magneto-optic rotation spectrum ofthe sample constituent to be detected.
 3. The method of claim 2including the step of, changing the optical passband frequencies of thelight passed by the filter to the optical detector for detectingmagneto-optic rotation of different components of the sample.
 4. In anapparatus for analyzing a sample by detection of magneto-optic rotation,means for probing a sample of matter to be analyzed with a light beamhaving light of a first polarization, means for applying a magneticfield to the sample with a substantial component of applied magneticfield being directed along the direction of the path of propagation ofthe probing light beam within the sample to produce magneto-opticrotation of the polarization of the light by the sample from thedirection of the first polarization to the direction of a secondpolarization, said magnetic field comprising an alternating magneticfield, whereby the alternating magnetic field produces a modulation ofthe magneto-optic rotation at a frequency which is a multiple of thefrequency of the applied alternating magnetic field, means forgenerating a reference signal at a frequency which is a multiple of thefrequency of the applied alternating magnetic field, means for analyzingthe polarization of the light emerging from the sample as affected bythe sample to separate the emerging light of tje first polarization fromthe emerging light of the second polarizaiton, means for detecting theseparated light of one of said polarizations to obtain a compositeelectrical signal having a time varying magneto-optic rotationelectrical component of a frequency which is a function of the frequencyof the modulation of the magneto-optic rotation and of an amplitudewhich is responsive to the quantity of material within the sample whichis magneto-optically active within the band of optical frequencies oflight incident of said detector, means for correlating the compositeelectrical signal against said reference signal to obtain an outputproportional to the time varying magneto-optic rotation electricalcomponent separated from the remainder of the composite electricalsignal, and means for measuring the amplitude of the output to obtain ameasure of the quantity of material within the sample which ismagneto-optically active within the band of optical frequencies of thelight detected by the said detector.
 5. The apparatus of claim 4 whereinthe probing light beam has a first optical bandwidth broader than themagneto-optic rotation band of a component to be detected in the sample,and including means for filtering the light emerging from the sample ata point between the sample and said light detector means for passing tosaid light detector means, only light within an optical passbandnarrower than the bandwidth of the incident probing light andencompassing substantially only the magneto-optic rotation spectrum ofthe sample constitutent to be detected.
 6. The apparatus of claim 5including means for changing the optical passband of frequencies of thelight passed by said filter to said light detector for detectingmagneto-optic rotation of different components within the sample.
 7. Theapparatus of claim 4 wherein said means for detecting the compositeelectrical signal against said reference signal includes synchronousdetector means for synchronously detecting the composite signal againstthe reference.
 8. The apparatus of claim 4 wherein said means fordetecting the composite electrical signal against said reference signalcomprises a phase sensitive detector means.
 9. The apparatus of claim 4wherein said means for detecting the composite electrical signal agaInstsaid reference signal comprises, counter means for counting in onedirection during certain time periods of the reference quantity and forcounting in the opposite direction during remaining periods of thereference quantity and means for subtracting counts in one directionfrom the counts in the other direction to derive a difference count,said difference count corresponding to an integration of the timevarying electrical magneto-optic rotation signal component which isseparated from the remainder of the composite electrical signal.